question about comparator

[wanily1983]

HI, all guys
i read some notes about comparator, but i am puzzled about these words "while some comparators may have an open collector output with grounded emitter. A reduced value for the pull-up resistor, providing increased current, will yield improved switching speed and noise immunity, but at the expense of increased power dissipation."
why reduce the pull up resistance can improved switching speed and noise immunity?
hope for ur reply! thanks in advance!

 

[vdaniel]

When your open collector switches off, the stray output
capacitance C will be charged trough the R pullup resistor
in the approximetely R*C time. So the less resistance, the
shorter rising time.
The noise immunity of any line depends of its resistance to
gound or decoupled power line. So the smaller pullup
resistor value, better the noise imunnity.
Varuzhan

 

[Alles Gute]

Hi,
I agree that speed improvement is due to reduced RC time constant. It's true that the noise immunity of any line depends of its resistance to gound or decoupled power line. But I don't see why "So the smaller pullup resistor value, better the noise imunnity."
I think the improvement on noise imunnity is due to the increased current. You can always spend more power to increase noise imunnity.

 

[vdaniel]

You can consider the electromagnetics wave noises as a current
source for your line, and the line impedace as a load resistor for
it. Vnoise=Inoise*Rline
To decrease noise you can decrease the I value, by using shields
or shorter line and decrease the R, which in our case is the pullup
resistors value.
Good noise immunity of the CMOS logic is because in static its
output is connected to the ground or power line through small (several
Ohms) resistance of one of the output MOS transistors channel.
If you want to have a good noise immunity without a small resistance
and accordinlgy a large power consuming, use a comparator
with totem-pole output, for example LMV7219.
Varuzhan

 

[Kral]

Alles Gute,

When the comparator output is in the "0" state, the pull-down transistor is on, and presents a very low impedance to the outside world. For an open collector (drain) comparator in the "1" state, the impedance presented to the outside world is the pullup resistance. Capacitively coupled noise "sees" this impedance. So the lower the pullup resistance, the more attenuation the capacitively coupled noise will see, and the less susceptible the output will be to capacitively coupled noise.
Regards,
Kral

 

[wanily1983]

thanks for all you reply, i really appreciate it.
i am still puzzled with the noise question. could you give some picture to show where the noise is from and the relation between the pull-up resistor and noise imunnity.